Motor oils are used in internal combustion engines for lubrication of movable parts in order to reduce the friction and wear of metal surfaces which are moved with respect to one another. The motor oil is subjected to multiple aging processes necessitating that the oil be changed after a certain period. It is of great interest to provide a sensor device and a method which are able to detect the state of the oil in an internal combustion engine that is in operation.
The state of the motor oil is influenced in various ways by numerous aging processes. The effect of some aging processes on the characteristics and composition of the motor oil is known. In addition, these aging processes may be identified by certain processes in an engine or devices connected to the engine. Conversely, possible incorrect settings or defects in the engine or in the connected devices may be detected from the state of the oil.
Two aging effects are the waste heat from the engine and the inflow of atmospheric oxygen, which result in oxidation and acidification of the oil. The oxidation results in increased viscosity of the oil. Determination of the viscosity thus allows a determination of the degree of oxidation. Since carboxylic acids are produced by oxidation of the motor oil and the quantity of carboxylic acids is therefore a function of the degree of oxidation, the proportion of these acids in the oil may also be detected via the viscosity. However, this very simple test method does not take into account additional effects which contribute to an increase in the viscosity of the oil without changing the acid content, such as a clogged exhaust gas aftertreatment device, for example, which increases the soot content in the engine.
In addition, the quantity of blow-by gases is not detected, since these result only in an increase in the proportion of acid, which cannot be directly detected via the viscosity.